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Why platforms stopped confirming emails
A search box that answers "this email has an account here" is a free account-enumeration tool for anyone with a list of addresses. Most large platforms closed that door deliberately.
So direct email-to-account lookup is largely gone. What remains works indirectly, and the honest answer is that the yield is lower than it used to be.
What still works, ranked
The local part of the address is the strongest pivot. If the address is firstname.lastname@, try that exact string as a handle on the platforms you care about, including the dot and underscore variants.
Gravatar and similar avatar services publish a profile keyed to an email hash for anyone who registered one. It is a small surface, but when it hits, it is a direct link.
Breach-notification services will tell you whether an address appeared in a known breach. That tells you where the person had accounts, not which accounts they still use — and the ethics of using breach data warrant a moment of thought before you do.
Exact-phrase search as a last resort
Searching the full address in quotes finds the places where someone published it themselves: support forums, marketplace listings, public profiles. That is legitimate public data, and it is the most defensible route.
Do not expect a full picture. Most people keep their address private, and the absence of results says nothing about whether accounts exist.
If you are doing this about yourself
Check whether your address is discoverable through Gravatar, your own forum history, or old marketplace listings. Removing those is usually a ten-minute job with a lasting privacy payoff.
Then use a distinct address for anything sensitive going forward, so that the next search starts from nothing.